Why do large veins have valves

Although arteries and veins differ structurally and functionally, they share certain features. Figure 1. The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colors show the relative levels of oxygen concentration. Different types of blood vessels vary slightly in their structures, but they share the same general features.

Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure Figure 2. Each type of vessel has a lumen —a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins.

Figure 2. By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance.

In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity.

The walls of arteries and veins are largely composed of living cells and their products including collagenous and elastic fibers ; the cells require nourishment and produce waste. Further, the walls of the larger vessels are too thick for nutrients to diffuse through to all of the cells. Since the pressure within arteries is relatively high, the vasa vasorum must function in the outer layers of the vessel or the pressure exerted by the blood passing through the vessel would collapse it, preventing any exchange from occurring.

The lower pressure within veins allows the vasa vasorum to be located closer to the lumen. The restriction of the vasa vasorum to the outer layers of arteries is thought to be one reason that arterial diseases are more common than venous diseases, since its location makes it more difficult to nourish the cells of the arteries and remove waste products. There are also minute nerves within the walls of both types of vessels that control the contraction and dilation of smooth muscle.

These minute nerves are known as the nervi vasorum. Both arteries and veins have the same three distinct tissue layers, called tunics from the Latin term tunica , for the garments first worn by ancient Romans; the term tunic is also used for some modern garments.

From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa.

Table 1 compares and contrasts the tunics of the arteries and veins. The tunica intima also called the tunica interna is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the collagenous fibers beneath is one of the primary causes of clot formation.

Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels.

Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called endothelins that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension high blood pressure and cardiovascular disease. Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue.

The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of areolar connective tissue that consists primarily of elastic fibers to provide the vessel with additional flexibility; it also contains some collagenous fibers to provide additional strength.

In larger arteries, there is also a thick, distinct layer of elastic fibers known as the internal elastic membrane also called the internal elastic lamina at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch.

It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen. Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls.

The tunica media is the substantial middle layer of the vessel wall see Figure 2. It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets.

Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively.

Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.

These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction, depending upon the nature of the neurotransmitter and receptors located on the target cell. Parasympathetic stimulation does trigger vasodilation as well as erection during sexual arousal in the external genitalia of both sexes. Nervous control over vessels tends to be more generalized than the specific targeting of individual blood vessels.

Local controls, discussed later, account for this phenomenon. Seek additional content for more information on these dynamic aspects of the autonomic nervous system. Hormones and local chemicals also control blood vessels. Together, these neural and chemical mechanisms reduce or increase blood flow in response to changing body conditions, from exercise to hydration.

Regulation of both blood flow and blood pressure is discussed in detail later in this chapter. The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also binds the tunica media to the inner and outer tunics.

Along with the collagenous fibers are large numbers of elastic fibers that appear as wavy lines in prepared slides. Separating the tunica media from the outer tunica externa in larger arteries is the external elastic membrane also called the external elastic lamina , which also appears wavy in slides.

Although most veins take blood back to the heart, portal veins carry blood between capillary beds. For example, the hepatic portal vein takes blood from the capillary beds in the digestive tract and transports it to the capillary beds in the liver. The blood is then drained in the gastrointestinal tract and spleen, where it is taken up by the hepatic veins and blood is taken back into the heart.

Since this is an important function in mammals, damage to the hepatic portal vein can be dangerous. Blood clotting in the hepatic portal vein can cause portal hypertension, which results in a decrease of blood fluid to the liver. Veins are classified in a number of ways, including superficial vs. Privacy Policy.

Skip to main content. Cardiovascular System: Blood Vessels. Search for:. The Venous System. Venules Venules are small blood vessels in the microcirculation that connect capillary beds to veins.

Learning Objectives Describe the venules of the venous system. Inform now! Lovely legs. Phlebologie ; Your doctor makes the diagnosis and decides on the therapy. If necessary, your doctor can prescribe wound therapy products. The patient is advised by trained personnel e. After this, the patient receives products that are customised to meet their individual needs. A quick overview: The venous system. If only one of these factors is disturbed, venous flow may become impaired: Heart and breathing: The different pressure ratios in the two halves of the heart, as well as the processes of inhalation and exhalation, create a suction effect that supports the blood flow to the heart.

The venous valves: The venous valves work like check valves that allow blood to flow only towards the heart: They are crucial for the blood to overcome the height from the leg veins to the heart. The calf muscle pump: When the foot and leg muscles are working, for example when running, the muscle pump is active and stimulates the venous return flow. Vein tension: The vessel wall serves as a source of counter-pressure for the blood and ensures that venous pressure does not rise any further.

The venous valves: Check valves in the veins Thanks to these venous valves , the blood in the veins flows in only one direction : back to the heart. Here are the hidden dangers of untreated varicose veins and what you can do about it. The danger of DVT occurs when a clot breaks off and circulates through your blood where it can damage your organs.

Pulmonary embolism, for example, is most often the result of a blood clot from the legs that travels to and lodges in the lungs, blocking a major artery. It can cause chest pain, shortness of breath, and a cough that brings up pinkish-red fluid.

Chronic Venous Insufficiency Varicose veins commonly cause a condition known as chronic venous insufficiency CVI , which occurs when blood pools in the veins of your legs, making it difficult for blood to circulate from your legs to your heart. The inadequate circulation often causes fluid to accumulate in the legs, a condition known as edema. People with CVI often experience pain, cramps, and weakness in their legs.

It may affect your mobility and reduce your quality of life. Phlebitis Varicose veins often cause a condition known as phlebitis, which is when your veins become inflamed. When compression stockings fail to improve symptoms and are ineffective, minimally invasive procedures are available for treatment. Endovenous laser therapy is a common approach that utilizes laser energy to close the affected vein.

Activities that improve blood flow, like being physically active, may reduce your risk. The important thing to know is that if you have varicose veins, you have options to treat them and prevent potentially dangerous complications. It is winter, the weather is cold and blustery, and we are wearing long pants or tights that cover our legs.

If you are already looking forward to summer sun, bathing suits, shorts, and short skirts, now is the perfect time to begin treatment for varicose and spider veins. Your varicose and spider veins did not develop overnight and they cannot be corrected overnight.

Treatment usually requires a series of appointments over a few weeks or months to achieve the best results. There are many advantages to treating varicose veins in the winter months.

Compression stockings can be hot in the warmer months, but are nice and cozy during the winter. The legs are covered up anyway, so no worries about the fashion statement compression hose might make!

There can be some bruising and discoloration during treatment, which will be hidden by winter clothes. Your winter wardrobe will allow your legs time to heal and time to reveal a noticeable difference in the appearance of your legs. Your legs should not be exposed to the sun during treatment. The tiny incisions required by some treatments heal best when they are not exposed to the sun. The earlier you begin treatment, the more likely you will have great-looking legs to show off when summer rolls around.

Winter treatment will result in less painful, less swollen legs, which will allow you to be more physically active in the summer months. If you are ready to say good bye to your varicose veins or spider veins, call The Vein Specialists at to schedule an appointment with one of our doctors to discuss your treatment options.

A baby bump is just one sign of pregnancy. Rapidly changing hormone levels contribute to many physical changes in your body including fatigue, breast tenderness, nausea, and constipation.

Not every woman experiences all of these side effects. Some will have one or two and others will have more.